Secondary batteries, e.g., lithium secondary batteries and nickel hydride batteries, have become increasingly important in recent years as power sources mounted in electricity-driven vehicles and as power sources installed in personal computers, portable terminals, and other electrical products. In particular, lithium secondary batteries (typically lithium ion batteries), which provide a high energy density at a low weight, are expected to be preferentially used as vehicle-mounted high output power sources.
A lithium secondary battery contains electrodes that have a structure in which a material capable of reversibly incorporating and discharging the lithium ion, which acts as the charge carrier, is borne on an electroconductive member (electrode current collector), and investigations are underway into electrode active materials in order to realize even higher energy densities and higher outputs. Lithium-containing complex oxides having a layered rocksalt-type crystalline structure and lithium-containing complex oxides having a spinel-type crystalline structure are examples of the positive electrode active materials used in the positive electrodes of lithium secondary batteries. Examples which contain lithium (Li) and at least one transition metal element are lithium-nickel complex oxides, lithium-cobalt complex oxides, and lithium-manganese complex oxides, and also lithium-nickel-cobalt-manganese complex oxides in which atoms of each of nickel, cobalt, and manganese are located at non-lithium metal sites in the aforementioned crystalline structures. Attention has been directed to the lithium-nickel-cobalt-manganese complex oxides in particular because they exhibit an excellent thermal stability and provide a high energy density. As prior art related to such positive electrode active materials for lithium secondary batteries, various lithium-containing complex oxides having a layered rocksalt-type crystalline structure are disclosed in Patent Documents 1 and 2.